The Tenth Doctor likened regeneration to death and then rebirth, and was understandably afraid to regenerate. He was given the warning that four knocks would signal his impending regeneration and indeed, Wilfred Mott knocking inside the radiation chamber meant the Tenth Doctor was on his way out. Holding off the regeneration for as long as he could, he took the time to visit each of his companions, helping them out the best way he could, before finally returning to his TARDIS and stating the line above, regenerating in a massive explosion of energy. That's what happens when you hold it in!